The study of the
uppermost section of the Early Cretaceous Agrio
Formation in northern Patagonia (Neuquén Basin)
where dinosaur tracks assigned to cf. Therangospodus pandemicus are exposed
(tracksites I and II) evidence mixed marginal marine siliciclastic-carbonate
deposits. The succession was divided in two intervals. A lower one containing
theropod tracks, recorded on top of subtidal oolithic limestones with tiny wave
ripples suggesting shoreline fluctuations and subaerial exposure. Tidal influence
is recognised by fining upward and prograding cycles starting with subtidal
carbonates and ending with fine-grained siliciclastic deposits at the top, or rarely
laminites. Dolomitization affects subtidal deposits generated in an alkaline
media stressful for tracemakers. Intertidal facies include abundant
heterolithic deposits, coquinas composed of gastropods encrusted by
multilayered bryozoans and muddy levels with incipient mud cracking.
Invertebrate ichnofossils recognized from tidally dominated deposits include Arenicolites, Kouphichnium, and Rhizocorallium. The upper interval is a transgressive–regressive
cycle that starts with dark shales, deficiently oxygenated, and covered by
prograding sandstones and finally sand flat deposits. This interval contains Gyrochorte, Hillichnus, and Ophiomorpha
documented in wave-influenced sandstones. Dinosaur tracks as well as Hillichnus, attributed to tellinoid
bivalves, and Kouphichnium assigned to
xiphosurans, imply the activity of producers rarely recorded previously as body
fossils in marginal marine deposits of southern South America. Previous
paleogeographic schemes are questioned by our analysis, which shows evidence of
extremely shallow and tide-controlled sedimentation, sometimes with subaerial
exposure, with high cyclicity related to a marginal marine depositional setting
and lack of significant erosion by the overlying unit, as traditionally was
suggested.